Uplink multi-user access communication can be implemented by different multiple access technologies, such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA) and Space Division Multiple Access (SDMA). Excellent access performance can be provided by using the CDMA technology to implement the uplink multi-user access communication, and the CDMA technology has been used by multiple wireless communication standards.
For an access process using the CDMA technology, multiple access terminals firstly use, respectively, a spreading sequence of a certain length (e.g., a spreading sequence of a length L consisted of L elements, herein the elements may be numerical symbols) to spread data symbols obtained by modulating data to be sent in an amplitude and phase modulation (such as Quadrature Amplitude Modulation (QAM)) mode. The spreading refers to a process in which each of the modulated data symbols is multiplied by each element of the spreading sequence to form a data symbol sequence with the same length as that of the spreading sequence used. In this process, each of the modulated data symbols (e.g., constellation point symbols corresponding to the data to be sent which has been modulated in the QAM mode) is multiplied by each symbol of the spreading sequence of the length L, such that each of the modulated data symbols is spread to be a data symbol sequence with the same length as that of the spreading sequence used, i.e., each of the modulated data symbols will be spread to be L symbols, which is equivalent to the case in which each of the modulated data symbols is carried by the spreading sequence of the length L, respectively. Then, the data symbol sequences, obtained after spreading process, of the multiple access terminals may be sent on the same time-frequency resources. Finally, after wireless propagation of the spread signals of the multiple access terminals, a base station receives a superimposed signal, and the base station can separate useful information of each of the terminals from the received superimposed signal through the multi-user reception and detection technology.
The CDMA belongs to the category of spread spectrum communication. Because the data symbols modulated by the terminals will be spread to be L symbols after being spread using a spreading sequence of the length L, if transmission time of the L spread symbols is to be equal to transmission time of the data symbols prior to the spreading, then the bandwidth for transmitting the L spread symbols is spread by a factor of L, thus a spreading sequence is often called a spread spectrum sequence.
The spread symbols of the access terminals may be transmitted through the multi-carrier technology (such as Orthogonal Frequency Division Multiplexing (OFDM) and Filter-Bank Multi-Carrier (FBMC)). Combination of the code division multiple access with the multi-carrier technology is the Multi Carrier-Code Division Multiple Access (MC-CDMA) technology.
In the CDMA technology, the spreading process of a transmitter is relatively simple: each modulated data symbol is multiplied by each symbol of a spreading sequence of the length L to obtain the L spread symbols, and then the L spread symbols are transmitted through the single-carrier technology or the multi-carrier technology; while a receiving process of a base station receiver is relatively complicated. How does the base station receiver separate the useful data information of each of the terminals from the superimposed signal accurately to ensure the multiple access performance of CDMA system is the key to the CDMA system. This involves two aspects, a spreading sequence and a receiver, herein selection of a spreading sequence is performance basis and the design of the receiver is performance assurance.
In order to acquire the excellent multiple access performance, a good cross-correlation is needed between the spreading sequences used by different terminals. If the single-carrier code division multiplexing technology is used, then the spreading sequences used by the terminals further need to have a good auto-correlation to confront the impact of multipath delay spread. While the multi-carrier code division multiplexing technology can rely on the multi-carrier technology to confront the impact of the multipath delay spread, and the design of the spreading sequence may consider emphatically the cross-correlation properties which is propitious for separation of multi-user information by the receiver.
On the basis of the design of a spreading sequence, the base station can separate the multi-user information using the multi-user reception and detection technology with high performance, such as the successive interference cancellation (SIC) reception and detection technology, the complexity of which, however, is relatively high, to obtain the excellent multiple access performance.
The selection and design of a spreading sequence is an important aspect of CDMA technology. Direct Sequence-Code Division Multiple Access (DS-CDMA) technology, which is a commonly-used technology in the CDMA technology, has been used as the uplink multi-user access technology for multiple wireless communication standards and systems. Its spreading sequence is a simple binary pseudo-noise (PN) real sequence. Moreover, the DS-CDMA based on a binary pseudo-noise real sequence is applied to the MC-CDMA technology as well. The binary pseudo-noise real sequence can also be called a binary pseudo-noise sequence, a value of each element or symbol of which is typically represented as 0 or 1 or as a bipolar sequence. That is, 0 is represented as +1 and 1 is represented as −1, or 0 is represented as −1 and 1 is represented as +1.
The design of a spreading sequence further needs to consider the length of the spreading sequence. The longer the spreading sequence, the more easier it will be to ensure low cross-correlation between the spreading sequences used by different access terminals, and the more easier it will be to select more sequences with low cross-correlation, thereby supporting simultaneous access of more terminals. If the number of the terminals having access to the system simultaneously is greater than the length of the spreading sequence, then it is believed that the system is in an overload status.
Supporting simultaneous access of a large number of users to the system for communication is an important requirement for the future wireless communication, which may be implemented by designing a multi-user access communication system with better overload capability based on the code division multiple access. Decreasing communication delay is another important requirement for the future wireless communication, which may be implemented by designing a multi-user access communication system with grant-free access characteristic based on the code division multiple access.
From the perspective of multi-user information theory, using a non-orthogonal multiple access mode in the uplink can obtain a greater system capacity or marginal throughput than using an orthogonal multiple access mode. Therefore, in order to provide a flexible system design and support simultaneous access of more users, different access terminals can use non-orthogonal spreading sequences. Since the spreading sequences of the different access terminals are not orthogonal to each other, the reception and detection performance of each of the terminals will become worse as the number of the terminals having access to the system simultaneously increases. When the system overloads, interference between multiple users will become more serious.
In the existing technology, the code division multiple access (CDMA) technology uses a spreading sequence based on a binary pseudo-noise real sequence, the length of which is relatively long. Thus, when a large number of user terminals have access to the system or when the system overloads, the performance will become worse when a traditional receiver (such as a RAKE receiver) is used. While the reception and detection complexity will be very high and the delay will be large when an interference cancellation receiver (such as a receiver using the SIC technology) is used. If a binary pseudo-noise real sequence of a shorter length is used, then low cross-correlation between the sequences cannot be guaranteed. When a large number of user terminals have access to the system or when the system overloads, serious interference between multiple users will be caused, further influencing the multi-user reception and detection performance and multi-user access communication performance.